Carboxylic acids are important chemicals of commerce. They appear as desired or contaminating constituents of a wide range of aqueous process streams. Historically, they were produced from animal fat or vegetable oil sources or from petroleum sources in substantially nonaqueous systems. More recently, they have been identified among the most attractive products for manufacture from biomass (e.g., corn starch) by fermentation. In these more advanced processes, the carboxylic acid is generated as a dilute solution in an aqueous fermentation broth. Acetic acid is recovered commercially from dilute aqueous solutions by distillation or by extraction with solvents such as isopropyl acetate, other esters, or ethers. Aqueous solutions are created during the manufacture of adipic acid. Citric acid is recovered from fermentation broths commercially by solvent extraction with high-molecular-weight tertiary amines (e.g., tridecylamine) in a diluent composed of a hydrocarbon (e.g., kerosene) and an alcohol (e.g., n-decanol). Citric acid commands a substantial market, which is increasing as detergent manufacturers switch to citric acid as "builder". Lactic acid (raw material for biodegradable plastics), succinic acid, malic acid, fumaric acid, and other carboxylic acids which may be manufactured on a large scale by fermentation of biomass are creating considerable interest in solvent extraction as a means of recovery. Carboxylic acids are also stable oxidation products and frequently appear as by-products or contaminants in aqueous and organic waste streams.
There are numerous current and potential industrial and environmental applications where it is desirable to recover these and other carboxylic acids from aqueous solutions. Examples include the production of citric acid and other acids by fermentation (Lockwood, 1979.sup.1 ; Busche, 1985.sup.2) and removal and recovery of carboxylic acids from aqueous waste streams. (All references noted herein are listed below in a section of the specification entitled "References.") For volatile carboxylic acids, such as acetic, distillation and azeotropic or extractive distillation are alternatives, along with solvent extraction and adsorption (King, 1983.sup.3 ; Kuo et al., 1987.sup.4). For low-volatility carboxylic acids, e.g., dicarboxylic acids and hydroxycarboxylic acids, distillative processes are expensive and often cannot isolate the desired acid.
For acids such as citric and lactic, the classical approach for recovery from a fermentation broth has been to add calcium hydroxide to form the calcium salt of the carboxylic acid, to which an acid such as sulfuric is added to liberate the free carboxylic acid. This approach consumes chemicals (e.g., lime and sulfuric acid) and produces a waste salt stream. Consequently, this method is falling out of favor.
B. Urbas, in U.S. Pat. Nos. 4,405,717.sup.5 and 4,444,881.sup.6, teaches a process for recovering acetic acid, lactic acid, butyric acid and citric acid directly from fermentation broths. This process involves converting the acid to a calcium salt and then adding a tertiary amine carbonate (especially tributylamine carbonate) to give a trialkylammonium salt of the acid and calcium carbonate. The trialkylammonium carboxylate is heated to give the acid and the corresponding trialkylamine. This process has the disadvantage that it generates calcium carbonate, a solid waste that needs to be disposed of or heated to high temperatures in a kiln to convert it back to calcium oxide. Also in these patents, there is a preference for higher molecular weight amines and the use of distillation to remove volatile acids from the less volatile amines.
Solvent extraction is often effective for recovery of these low-volatility carboxylic acids from aqueous solution. Reactive, basic extractants, e.g., tertiary amines or phosphine oxides, can be used to gain greater solvent capacity and selectivity with respect to water and other species. A process developed by Miles, Inc. (Baniel et al., 1981.sup.7) for recovery of citric acid from fermentation solutions uses a solvent composed of a tertiary amine extractant in a hydrocarbon diluent with an alcohol modifier. This extractant is regenerated by back-extraction of the acid into water at a higher temperature. Back-extraction following a shift in diluent composition, achieved, e.g., by distillation, is another possibility for regeneration, and can be combined with a swing of temperature (Tamada and King, 1990.sup.8,9, and Baniel et al., 1981.sup.7). The overall degree of concentration relative to the feed that can be achieved by these methods is limited by the extent to which the distribution equilibrium for the carboxylic acid can be changed between forward and back-extraction and also by the feed concentration itself.
Ion exchange and adsorption have also been employed in carboxylic acid recovery schemes. U.S. Pat. No. 4,720,579 to Kulprathipanja.sup.10 discloses the use of styrene-divinylbenzene resins to adsorb citric acid with regeneration by water or by a mixture of acetone and water. Similarly, Great Britain Patent No. 2,064,526A.sup.11 discloses the use of adsorbents containing pyridyl functional groups combined with regeneration by leaching with an organic solvent such as an alcohol or a ketone. U.S. Pat. No. 4,924,027 to Kulprathipanja and Strong.sup.12 discloses adsorption of citric acid by adsorbents containing tertiary amine or pyridyl functionalities (including Bio-Rad AG3-X4A and AG4-X4), with regeneration using an aqueous solution of sodium, potassium or ammonium hydroxide, yielding the respective sodium, potassium or ammonium citrate. Treatment of these citrates with a strong acid would yield the free citric acid form. In each of these solutions the citric acid is adsorbed from an aqueous solution below the pK.sub.a1 of citric acid.
Many fermentations to produce carboxylic acids operate most effectively at pH&gt;pK.sub.a1 of the acid, where the acid exists primarily as the carboxylate salt. One example is lactic acid, for which pK.sub.a1 =3.86.sup.13 and which is produced by fermentation at pH values typically in the range of 5 to 6..sup.14 For the types of processes under consideration here, the driving force for separation is the concentration of the un-ionized form of the acid..sup.8,9,15,16,17 Therefore, a compromise is needed between a high pH (above the acid pK.sub.a1) for the fermentation and a low pH (below the acid pK.sub.a1) for the separation. A method of recovering the free acid from solution at high pH would be valuable.
Several researchers have used carbon dioxide as an acidulent during solvent extraction of carboxylic acids from the corresponding carboxylate salt solutions. Yates.sup.18 describes a process whereby a carboxylate salt solution is contacted with a water-immiscible polar organic solvent in the presence of carbon dioxide. This patent also describes anion exchange of carboxylate anion with an anion-exchange resin in the bicarbonate form. In this scheme, recovery of the acid is accomplished by regeneration of the resin using a water-containing organic solvent in the presence of carbon dioxide. The carboxylic acid is extracted into the organic solvent, and the resin is reloaded with bicarbonate anion.
Baniel et al..sup.19 describe a process for recovery of lactic acid from aqueous lactate solutions using reactive extraction with a water-immiscible trialkylamine in the presence of carbon dioxide. Lightfoot et al..sup.20 similarly describe a process for recovery of lactic acid from aqueous calcium lactate solutions using reactive extraction with a long-chain tertiary or secondary amine in a water-immiscible organic solvent in the presence of carbon dioxide. Hu and Adeyiga.sup.21 presented an analogous study of reactive extraction of formic acid from solutions of sodium formate.
Several advantages exist for using basic solid sorbents, rather than liquid extractants, as complexing agents. Sorption can avoid the problems of emulsion formation and aqueous-phase contamination due to the solubility of the complexing agent and/or diluent(s) that exist with extraction. Although precipitation of low-solubility salts (e.g., CaCO.sub.3) could be a concern with fixed beds during sorption, an appropriate choice of cation (e.g., Na.sup.+) can avoid this problem for dilute carboxylate solutions.
Historically, as stated above, the conventional technique for recovering non-volatile carboxylic acids from aqueous solution has been precipitation of the calcium carboxylate salt. Both citric and lactic acids are recovered from fermentation broths by this technique..sup.22 Drawbacks to this approach include substantial energy and chemical costs, loss of product acid because of the solubility of the calcium salt, and production of relatively impure CaSO.sub.4.
Recovery technology using reversible chemical complexation with polymeric sorbents having amine functionalities can reduce energy consumption substantially. If a method of regeneration allowing recovery and reuse of all agents is utilized, such processes can also avoid production of waste salts and net consumption of chemical agents. Previous researchers.sup.7,15,16,23-28 have shown that extraction and adsorption by reversible chemical complexation are effective for recovery of carboxylic acids from dilute aqueous solutions. Amine-based extractants and solid sorbents sustain uptake capacity for carboxylic acids from solutions at pH above the pK.sub.a1 of the acid, where the acid exists mostly as the carboxylate anion.
A shortcoming of using strongly basic complexing agents is that, if a carboxylic acid is removed from an unbuffered solution, the pH will rise sharply if substantial concentrations of strong-base cations (e.g., Na.sup.+, Ca.sup.2+) are present. This pH rise reduces the uptake capacity of the complexing agent and results in low percent recoveries of acid anion. A method is needed to maintain low pH, and thus to sustain the uptake capacity of the complexing agent during the acid recovery step.
One approach for supplying the necessary protons to convert the carboxylate salt into the corresponding acid is to acidify the salt solution directly with a strong acid (e.g., H.sub.2 SO.sub.4 or H.sub.3 PO.sub.4). Kulprathipanja et al..sup.10,12, for example, recovered lactic and citric acids using non-functionalized polystyrene-divinylbenzene sorbents and weak-base anion-exchange resins with acidification by addition of sulfuric acid. Seevaratnam et al..sup.29 recovered lactic acid from fermentation broths using adsorption and extraction coupled with acidification by addition of hydrochloric acid. In both of these cases, the pH of the fermentation broth must be adjusted with base if it is to be returned to the fermentor. Thus, one major disadvantage of recovering carboxylic acids by extraction or adsorption with strong-acid addition is that acid and base are consumed, and salts build up in the broth and must be removed. Additionally, competitive sorption can occur between the strong-acid anions and the carboxylate anions..sup.30
Cation exchange to replace the strong-base cations in solution with protons avoids the problem of introducing strong-acid anions (e.g., SO.sub.4.sup.2-, PO.sub.4.sup.3-) into solution. Cation exchange does not offer any solution to the problem of waste salt formation, however. When the cation-exchange resin is depleted of protons, it must be regenerated with a strong acid--most often H.sub.2 SO.sub.4 --resulting in the production of a sulfate salt waste stream.
Addition of a suitable buffer to the solution has the potential to prevent the pH-swing associated with acid recovery. This buffer would need to be added at a sufficient concentration to provide a large buffering capacity, without incurring detrimental effects to the microorganisms if it is applied directly to a fermentation. It should also have a pK.sub.a similar to the pH that one is trying to maintain and should not compete effectively with the carboxylic acid for the basic sites on the sorbent.
As can be seen from the foregoing, various methods used heretofore to recover carboxylic acids have presented limitations and thus offer opportunities for improvement.
One such improved process is described and claimed in U.S. Pat. No. 5,412,126, King et al..sup.31, and incorporated by reference herein. In this process, carboxylic acid is sorbed from an aqueous feedstock into an organic liquid phase or onto a solid adsorbent. The acid is freed from the sorbent phase by treating it with aqueous, low-molecular-weight alkylamine thus forming an alkylammonium carboxylate which is dewatered and decomposed to the desired carboxylic acid and the alkylamine.
Another variation, as described in Ind. Eng. Chem. Res. 1998, 37, 2996-3005, Husson and King.sup.32, also U.S. patent application Ser. No. 08/943,514, filed Oct. 3, 1997, both of which are incorporated by reference herein, is to sorb the carboxylic acid onto a solid basic adsorbent, then regenerate it by treating the sorbed mass with an organic solution of an alkylamine. The alkylamine-carboxylic acid complex thus formed is thermally decomposed to provide the desired acid and alkylamine.
It has now been found, however, that no matter which of the above indicated processes is used, it can be further improved upon and made more efficient by using carbon dioxide under pressure during the first step of the process in which the carboxylic acid containing feedstream is contacted with the acid-sorbing phase.
It is accordingly a general object of the invention to provide an efficient process for the recovery of carboxylic acids from aqueous solutions which neither consumes large amounts of chemicals nor generates waste chemical streams.
It is a further object to provide a process for the recovery of free carboxylic acid from an aqueous solution at a high pH.
It is yet another object of the invention to provide a process for the recovery of free carboxylic acid from an aqueous solution where the pH of the solution is greater than the pK.sub.a of the acid.
It is a still further object of the invention to provide an efficient process for the recovery of carboxylic acid where the process is carried out in the presence of carbon dioxide.